1. Field of the Invention
The present invention relates to an apparatus, a method, and a program for detecting a modulating parameter of a multiplex modulation system.
2. Description of the Related Art
There is an orthogonal frequency division multiplex (OFDM) signal as a type of multiplex modulating signals. The communication system using the OFDM signal uses a plurality of sub-carriers whose center frequencies are different from each other so that there provides a high frequency efficiency. In addition, it transmits transmission signals by multi-carrier signals thus exhibiting a property that is strong for interference. Therefore, the communication system using the OFDM signal has been actively applied to various kinds of communication systems such as radio LAN and the like (for example, Patent Document 1). As one of the radio LAN standard, it is a characteristic of the OFDM signal to have a structure in which a pilot symbol applied to synchronizing processing and a guard interval symbol for reducing the influence of multi-paths and for being applied to the synchronizing processing are added to an OFDM data symbol to be transmitted (Non-Patent Document 1).
FIG. 1A shows an example of the OFDM signal structure to which the present invention can be applied. As the basic structure of an OFDM signal frame, a pilot symbol aimed for synchronizing processing is arranged in the front and, following the pilot symbol, the type of communication signal and a plurality of OFDM data symbols as transmission data are arranged. In FIG. 1A, Np is the length of the pilot symbol, Ng is the length of the guard interval symbol, Nd is the length of the OFDM data symbol, and M is the number of the OFDM data symbols.
As shown in FIG. 1B, for synchronizing processing and in order to reduce the influence by multi-paths, a signal, which is a copy of a part of the OFDM data symbol to be transmitted, is added to the front of each OFDM data symbol as the guard interval symbol.
In conventional OFDM signal detection and demodulation processing, synchronizing and demodulation processing are performed on assumption that the length Np of the pilot symbol, the length Ng of the guard interval symbol, the length Nd of the OFDM data symbol, and the symbol number M of the OFDM data symbol per frame are known as information on a reception side.
In the OFDM communication system in which the modulating parameter is uniquely changed by a transmission side in accordance with the communication quality, it is important to detect the modulating parameter based on a received signal in the demodulation processing performed on the reception side. With this, data communication for informing a change of parameter becomes unnecessary and an increase in the data transmission capacity can be expected. Further, in a system for monitoring illegal radio waves, these parameters are generally unknown so that it is necessary to extract the parameter from the received signal.
As one of the conventional processing methods, there are two ways described in Non-Patent Document 2 in regards to synchronizing demodulation method under a condition that the above-described parameters are known. One is a method in which a PN-system pilot symbol is added to the front of an OFDM signal block and the principle of CDMA matched filter is used. From a result of the correlation value output (see Non-Patent Document 2, pp. 82-85), the method achieves frame synchronizing processing of the OFDM signal, and synchronizing and demodulation processing by detecting the phase error caused by radio wave propagation.
The other is synchronizing demodulation processing using the guard interval symbol. In this method, in a circuit shown in FIG. 2 (see Non-Patent Document 2, p. 57, FIG. 4.2, guard interval symbol type synchronizing circuit), two data windows for correlation processing, which have time delay difference for the length of the OFDM data symbol, are used for input signals, and synchronizing and demodulation processing are performed based on the correlation result. In FIG. 2, T is the OFDM block length without the guard interval symbol and Tg is the length of the guard interval symbol.
[Patent Document 1] Japanese Patent Unexamined Publication No. 2001-211137
[Non-Patent Document 1] IEEE Standard 802.11a-1999, p.12, 17.3.3 PLCP preamble (SYNC) FIG. 110-OFDM training structure
[Non-Patent Document 2] “OFDM System Technology and MATLAB Simulation Guide” by Ochi Hiroshi, pp. 53-71, published by TRICEPS, Inc.,
However, it is necessary in the first processing method disclosed in the above-described Non-Patent Document 2 to perform the correlation processing on assumption that the reception side knows the length and the system of the pilot symbol which is added to the transmission signal. Further, the second processing method disclosed in Non-Patent Document 2 achieves the effect of its action only when the length of the OFDM data symbol is known.
As describe above, the conventional OFDM modulation circuit is operable only under the condition where the modulating parameter of the OFDM signal is known and it does not properly operate when there is an input of signal whose modulating parameter of the OFDM signal is unknown.